A general hydraulic excavator includes a base machine and a working attachment attached to the base machine, the working attachment having a boom capable of being raised and lowered, an arm pivotably coupled to a tip of the boom, a bucket attached to a tip of the arm, a boom cylinder for raising and lowering the boom, an arm cylinder for pivoting the arm, and a bucket cylinder for pivoting the bucket. The boom cylinder is interposed between the boom and the base machine so as to raise the boom when extending, and the arm cylinder is interposed between the arm and the boom so as to pivot the arm in a crowding direction (the direction in which the arm approaches the boom) when extending.
In the base machine, there is installed a hydraulic circuit to extend and contract each of the cylinders. The hydraulic circuit includes a hydraulic pump which sucks hydraulic oil stored in a tank and discharges it, and a plurality of control valves interposed between the hydraulic pump and respective cylinders to switch a direction in which the hydraulic oil is fed from the hydraulic pump to the cylinder. Extending and contracting operations of the cylinders are performed by respective operations of the control valves.
Such a hydraulic excavator implements various types of work such as excavation by respective movements of the boom, the arm, and the bucket. For example, Patent Literature 1 indicates that desired excavating work is performed by use of a combined operation of a boom raising operation which is an operation of the boom in the raising direction and an arm crowding operation which is an operation of the arm in the crowding direction.
Conventional construction machines illustrated by the above hydraulic excavator require considerable power, for example, considerable engine horsepower, for rotating the hydraulic pump to extend and contract each cylinder, and reducing the required power is an important challenge. In particular, such excavating work as involves simultaneous execution of the boom raising operation and the arm crowding operation requires considerable horsepower for simultaneous extensions of both of the boom cylinder and the arm cylinder, and reducing the required horsepower is sought.
As means for preventing a vehicle body from being raised by an excavation reaction force during the combined operation, Patent Literature 1 discloses a technique of providing a supply oil passage bringing a rod-side chamber of the boom cylinder and a head-side chamber of the arm cylinder into communication with each other and a selector valve adapted to open and close the supply oil passage, wherein the selector valve is opened to permit hydraulic oil to flow from the head-side chamber of the arm cylinder into the head-side chamber of the boom cylinder only when the rod-side chamber of the boom cylinder rises to a given height or higher, thus automatically extending the boom cylinder; however, the technique is hardly effective in reducing the required power as described above.